Multi-pass laser resonators, also called multi-fold or folded laser resonators, are commonly used in lasers to achieve a long effective gain path while maintaining a short physical length for the resonator cavity. Although the folding of the beam path can occur in one, two or three dimensions, linearly folded multi-pass resonators have the advantage of being relatively easy to construct. Within the linear multi-pass configuration, complete free space, complete waveguide or hybrid operation can define the axes transverse to the beam path. While the use of a waveguide in the folded axis can constrain laser oscillation to a single mode, it is sometimes desirable to use free space propagation instead, due to the ability to more easily achieve a Gaussian beam in free space.
A multi-pass laser resonator, also referred to as a multi-pass optical cavity, may be formed by folding a stable single pass resonator one or more times via one or more mirrors. For example, FIG. 1A shows a stable single pass resonator 101 formed by two mirrors 103 and 105. Located between the two mirrors 103 and 105 is a gain medium 106 that causes an emission of electromagnetic energy that then builds up within the resonator 101. Due to the high reflectivity of the mirrors 103 and 105, most of the energy is contained within the resonator 101 and, as a result, an intra-cavity laser beam 107 is generated. The mirror 103, also commonly referred to as an output coupler, allows for a small fraction of the energy to leave the resonator in the form of output laser beam 109. The output laser beam 109 may be employed for a number of different uses, e.g., laser cutting, welding, marking, or any other use.
The intra-cavity laser beam 107 established between the mirrors 103 and 105 of the resonator 101 may oscillate in what is known as the fundamental mode of the resonator 101. The fundamental mode of the resonator 101 may be characterized, in part, by a particular beam shape in the transverse direction, i.e., in a direction that is perpendicular to the direction of propagation of the intra-cavity laser beam 107. For example, the fundamental mode of the resonator 101 may be characterized by a beam whose beam shape follows a Gaussian intensity distribution. As used herein, the radius of a Gaussian laser beam is defined to be the distance (from the center location of peak intensity in the beam) at which the intensity of the beam is reduced by a factor 1/e2. Furthermore, a waist w1 of a Gaussian beam occurs at the longitudinal position on the beam having the smallest radius. For example, for the stable resonator shown in FIG. 1A having one flat mirror 103 and one concave mirror 105 separated by a distance L1, the waist w1 of the intra-cavity Gaussian laser beam 107 occurs at the surface of the flat mirror 103. Furthermore in this configuration, the separation between the mirrors 103 and 107 define what is referred to as the path length of the inter-cavity laser beam 107. Thus, for the in-line resonator configuration shown in FIG. 1A, the physical length of the resonator is equivalent to the path length L1.
FIG. 1B shows another arrangement where the physical length of the resonator may be shorter than the path length of the inter-cavity laser beam 107. In FIG. 1B, a lengthening of the path length of the inter-cavity laser beam 107 may be achieved by including flat turning mirror 111 within the cavity. The effect of flat turning mirror 111 is to fold the path of the inter-cavity laser beam 107 without necessarily changing the nature of the stable resonator depicted in FIG. 1A. For example, in the folded configuration shown in FIG. 1B, the physical length of the resonator L2 is approximately half the path length L1 of the inter-cavity laser beam 107. As shown in FIG. 1B the laser beam 107 makes two passes through the gain medium 106 with the first pass represented by portion 107a of laser beam 107 and the second pass represented by portion 107b of laser beam 107. Because the inter-cavity laser beam takes two passes through the gain medium 106, the folded configuration may achieve a higher gain, and a correspondingly higher output power in the output laser beam 109.